The present invention relates to the field of semiconductor metrology tools and, in particular, relates to a tool that combines two complementary types of measurements into a single tool to reduce ambiguities in both types of measurements.
In the past, there have been developed a number of devices to measure the electrical parameters of a semiconductor. A typical semiconductor sample might have an oxide layer formed on top of a silicon layer. Additional oxide layers or polysilicon layers can be added on top.
In order to measure the electrical thickness of the layers in a non-contact fashion, a charge is placed on the sample. The capacitance is then measured. The tool can determine electrical thickness based on the equation:
telectrical=xcex5A/Cxe2x80x83xe2x80x83(1) 
where t equals the thickness, xcex5 is the electrical permittivity, A is the surface area and C is the capacitance. Permittivity xcex5 is related to the index of refraction (n) by the formula xcex5=n2xcex50, where xcex50 is the permittivity of free space. Also of interest is the xe2x80x9cdielectric constantxe2x80x9d xcexa=xcex5/xcex50 and the refractive index n2=xcexa.
One disadvantage of these electrical metrology devices is that the measurements can be skewed by factors that are either unobservable or are difficult to observe. For example, if the silicon substrate is contaminated with ions, this skews the voltage or capacitance measurements and, thus, the resulting calculations of electrical properties such as electrical thickness of the film.
Furthermore, the determination of at least some electrical properties of the film such as electrical thickness is dependent upon accurate knowledge of the dielectric constant or other inherent properties of the oxide layer. In a typical electrical metrology device, many of the parameters which cannot be measured are assumed to have a particular value. If this assumed value is not correct, the resultant determination of the characteristics of the sample will be incorrect.
Accordingly, it is an object of the subject invention to provide a system which provides multiple independent measurements in order to improve the accuracy of the evaluation of the sample.
In accordance with the subject invention, electrical measurements are improved by providing a separate, independent measurement of at least one of these electrical parameters. One possibility would be to use an optical metrology device to measure thin film thickness. One example of such a device is described in U.S. Pat. No. 5,798,837, issued Aug. 25, 1998, and assigned to the same assignee as herein. This tool combines a number of optical inspection modalities, including spectroscopy and ellipsometry. Each of these modalities is designed to measure composition characteristics of thin films, including thickness and index refraction and extinction coefficient.
If the measurement results from an optical metrology device are combined with the measurement results from an electrical metrology device, the analysis of sample parameters could be improved. For example, the optical metrology device can be used to measure the thickness or index of refraction of the thin film layer. This data can then be used to reduce ambiguities in the electrical measurements. To achieve this result, one might determine an optical thickness value using the optical measurement tool and then substitute that optical thickness value for the electrical thickness in equation 1 above to permit a more accurate determination of the dielectric constant of the sample.
Note that the measurement of the index of refraction is made at optical frequencies (1014-1015 hertz) while the permittivity (dielectric constant) is typically measured in the megahertz to gigahertz regime.
The subject combination can also be used to improve the evaluation of composition characteristics of the thin films. More specifically, the electrical measurement system could be used to determine the permittivity of the film and using that information as a known parameter (which is related to index of refraction) the additional compositional parameters including thickness and extinction coefficient can be determined based on the optical measurements.
In the above two approaches, one of the sets of measurements is used to determine one parameter which is then used as a known in the calculation of additional parameters using the other set of measurements. Another approach would be to combine all of the measurements into sets of equations which have multiple unknowns and a best fit solution is found using iterative type calculations such as least square fitting routines. Various other approaches for fitting the data are within the scope of the subject invention.